Transmissions allow the variation of a ratio of rotation speeds between an input and output. As such, they allow controlling the rotation speed at their output when the input speed is either larger or smaller than the output speed.
In many applications, the input speed is determined by a power band of a power source connected to the input of the transmission. Indeed, many power sources provide an optimal torque or power over only a limited range of rotation speeds. For example, this is the case wherein the power source is an animal or a human.
Typically, humans provide power for locomotion or other purposes through a repetitive movement. In this document, each cycle of the repetitive movement is referred to as a stroke. For example, for a human pedaling a bicycle, the stroke corresponds to a complete turn of the cranks and pedals with respect to the crank axle. In another example, if the human is propelling a wheelchair, the stroke includes the motion of each arm of the human as it grabs the wheel, pushes onto the wheel, releases the wheel and gets back to a position wherein the cycle starts over.
Because of anatomically caused geometric constraints, inertial effects and muscle physiology, among other causes, humans typically produce an optimal amount of power only over a limited range of stroke frequencies. In addition, in most motions, power is not uniformly produced over the duration of the stroke. For example, in the case of a bicycle, almost no propulsive power is produced when the leg of a cyclist is at the top or the bottom of a pedaling motion, or in other words around the maximal and minimal knee flexion. Also, in the case of a wheelchair, almost no power is produced at the beginning or at the end of the wheel propulsion cycle, while maximal force is exerted in the vicinity of the middle of the propulsion cycle wherein the arms of the user are in proximity to a top portion of the wheels.
In many situations wherein humans produce power, for example for locomotion, an apparatus receiving the motion includes a transmission for matching the relatively narrow range of frequencies of efficient strokes to the desired speed of locomotion. For example, to match a relatively narrow range of frequencies at which a human can pedal a bicycle, gears are provided such that for a relatively constant stroke frequency, the bicycle travels over a wide range of speeds.
However, in most bicycle transmissions currently available, the gears come in discreet steps. Accordingly, there is the need to either have a very large number of gears such that the bicycle can be pedaled efficiently at all speeds, or alternatively, there is a need for the human to produce a suboptimal power as a limited number of gears requires that the human produces strokes over a relatively broad interval of stroke frequencies.
The few continuously variable transmissions (CVTs) that have been proposed for bicycles are typically too heavy and too inefficient to be suitable to their intended use.
In addition, many solutions have been proposed to reduce the duration of phases in a stroke that contribute relatively little to power production. These phases are typically known as “dead spots”, and many systems have been conceived to reduce, as much as possible, their presence in the pedaling cycle. An example of such a system includes the use of elliptical chain rings on a bicycle that increase a velocity of the pedals in the vicinity of the dead spots such that the human pedaling the bicycle stays for a relatively small amount of time in the vicinity of the dead spots. Unfortunately, most of the proposed devices for reducing dead spots are relatively inefficient and typically fall into disuse after only a few years on the market.
An area wherein this problem is even greater is in the field of wheelchairs. Typically, an intended user propels a wheelchair through either direct contact with the wheel or through contact with a handrim. The handrim and the wheel are conventionally fixed with respect to each other. Accordingly, there is no means for adapting rotation speeds of the handrim or wheel to the requirements of the human in wheelchairs. In addition, dead spots in the strokes used to propel wheelchairs are also relatively important and typically the intended can better propel a wheelchair only through the development of technique and strength.
The above discussion is mainly concerned with human powered vehicles. However, similar problems are encountered in many other human power applications and in motor-driven applications.
Against this Background, there Exists a Need in the Industry to Provide a Novel Continuously Variable Transmission.
An object of the present invention is therefore to provide an improved continuously variable transmission.